Most houses are stick built, i.e., constructed of 2.times.dimensional structural lumber members and nails. This approach to building construction is slow and manpower intensive, requires a large supply of a limited commodity, and affords a limited number of structural shapes.
Structural Insulated Panels (SIPs) are increasingly being used in building construction as an alternative to the stick built approach. SIP construction employs two rigid faces on either side of a light insulating foam core. High strength bonding of the outer facings to the inner core forms a structural I-beam in the form of flat panels which are typically joined together by lumber and nails. The outer, opposed panel faces are typically formed from conventional building materials such as gypsum or cementous composites, plywood, oriented strand board (OSB), drywall, or other rigid construction boards from 1/4" to 3/4" thick. Panels formed from the aforementioned materials suffer primarily from two limitations. One shortcoming of these panels is their limited tensile strength which renders these types of panels unable to accommodate large transverse loads. Another limitation relates to the inability of these types of panels to environmentally isolate one side of the panel from the other. For example, panel facings comprised of the aforementioned materials as well as the inner foam core of the panel are not weather resistant and permit moisture to travel between the outer panel facings. This limits the use of these types of panels in outer walls and roofs in many building structures such as houses and office buildings.
The prior art discloses various approaches for increasing the strength of these structural insulated panels. One approach incorporates wood members in the panel to increase its strength. But panels strengthened in this manner are subject to moisture degradation and insect infestation when used on the outside of a structure. Applicant's U.S. Pat. No. 5,497,589 employs metal edges disposed about and securely attached to the panel's inner foam core and outer opposed facings affixed to the inner core. The metal strip around the peripheral edge of the panel increases the strength of the panel, eliminates the need for structural members such as studs which act as thermal conductors, and facilitates coupling between adjacent panels. Applicant's U.S. Pat. No. 5,628,158 increases the strength of joined panels by inserting a spline in facing grooved edges of connected panels. The spline includes an insulating core and a pair of outer facing metal strips extending the length of the spline. The metal-faced spline is affixed to the panel edges by means of a structural adhesive for securely connecting the two insulated panels. Still another approach to strengthening structural insulated panels is disclosed in applicant's allowed U.S. patent application Ser. No. 08/853,696, filed May 8, 1997, wherein metal strips are incorporated in the panel and are bonded to the insulating core and an outer facing. Another approach employs a metal facing on one or both surfaces of the structural insulated panel to increase panel strength and environmentally isolate one side of the panel from the other, but this substantially increases the cost of the panel and precludes use of the panel in many types of common structures.
Many structural insulated panels incorporate an aforementioned gypsum or cementous outer facing having a paper outer surface. The paper used on these panel facings is not treated, nor is it impregnated, and thus does not contribute to panel rigidity or strength. Most paper impregnation processes use plastics like phenolic which actually reduce the tensile strength of the paper making it impractical for use as a building material.
The present invention represents an improvement over the prior art by providing a low cost structural insulated panel having a plastic foam inner core and opposed outer facings comprised of conventional building materials which has a high tensile strength for withstanding large transverse loads and provides an environmental seal between its opposed inner and outer surfaces.